This invention relates generally to high-performance analog-to-digital (A/D) converters, and more particularly, to techniques for A/D conversion employing superconducting Josephson junction devices. There is a requirement for high-performance A/D conversion equipment in both military and commercial applications. Two important measures of A/D converter performance are speed, i.e., the sampling rate in samples converted per second, and resolution, as measured by the smallest increment of change that can be detected in an analog signal. Many applications require both high sampling rates and high resolutions. Conventional techniques employing semiconductor circuitry have not been able to satisfy this requirement.
A/D conversion using Josephson junctions has been described in the technical literature. John P. Hurrell et al. described one such technique in a paper entitled "Analog-to-Digital Conversion with Unlatched SQUID's," published in the IEEE Transactions on Electron Devices, Vol. ED-27, No. 10, pp. 1887-96 (October 1980). SQUID is an acronym for Superconducting Quantum Interference Device.
The theory of operation of SQUID's for use in A/D conversion is explained in detail in the Hurrell et al. paper, and only a simplified explanation will be provided here. Similarly, the theory of operation of Josephson junctions is now widely known, and has been the subject of discussion in many technical papers. For example, see B. D. Josephson, "Supercurrents through Barriers," Advan. Phys., Vol. 14, pp. 419-51 (1965), and other papers cited in the Hurrell et al. paper.
A Josephson junction has a current-voltage characteristic that includes a region in which the current increases rapidly from zero, with practically no corresponding increase in voltage across the device. A SQUID is a circuit including one or more Josephson junctions and one or more inductive loads. A single-junction SQUID includes a Josephson junction connected across an inductance. If a current is injected into one end of the inductance and the other end is grounded, the resulting characterics provide the basis for A/D conversion, as explained in detail in the Hurell et al. paper.
The most pertinent property of the single-junction SQUID, from the standpoint of A/D conversion, is to be found in the relationship between the magnetic flux in the SQUID and the value of the injected current. This flux-current relationship is a periodic and, depending on the circuit parameters chosen, a multi-valued function. The most significant aspect of the relationship is that the flux changes by a small quantum whenever the current increases by a small and precisely repeatable increment. This quantum of flux gives rise to a small but measurable voltage pulse across the junction. When the current is decreased, a flux quantum of opposite polarity is produced for each precise decrement of current, and a corresponding voltage pulse of opposite polarity is produced across the junction.
This property of the single-junction SQUID forms the basis for the A/D converter described in the Hurrell et al. paper. A signal to be converted from analog to digital form is introduced into the single-junction SQUID as a varying current. Each time the current increases or decreases by a predetermined increment, a measurable voltage pulse is generated across the junction. In this manner the single-junction SQUID functions as a quantizer. The resultant pulses are then detected and counted in one or more counters. The principal advantage of the arrangement is its near perfect linearity. Another advantage is its sensitivity. The current increment, which determines the resolution, can be made extremely small. The flux quantum is only 2.07.times.10.sup.-15 weber, and the current increment is given by this value divided by the value of the load inductance (measured in henries).
A simple, two-stage counter for detection and recording of the pulses from the single-junction SQUID quantizer is also desribed in the Hurrell et al. paper. The counter described is a unidirectional two-stage one, employing double-junction SQUIDS as the counting elements. A double-junction SQUID comprises two Josephson junctions and a center-tapped inductance. The end terminals of the inductance are connected to a terminal of each of the junctions, and the other terminals of the junctions are connected together to ground. A control current is injected across the inductance, and a gate current is injected at the center tap of the inductance.
As explained in detail in the Hurrell paper, the double-junction SQUID circuit is bi-stable if the currents are appropriately chosen and controlled. Basically, in each of its two stable states the circuit has a circulating current component that flows through both of the junctions and the inductance. The direction of the circulating current component determines which state the circuit is in. When the gate current is raised momentarily above a threshold level, one of the junctions generates a voltage pulse and the direction of the circulating current reverses. Subsequent pulses applied to the gate current toggle or reverse the state of the SQUID. Multiple circuits working on this principle can be connected in a cascade arrangement to operate as a binary scaler, counting the number of pulses from the quantizer.
Although the underlying theory of the SQUID A/D converter has been previously described by Hurrell et al., their device was not an ideal one in some respects. First, the Hurrell et al. quantizer employs a single-junction SQUID that produces pulses of two different polarites, depending on the direction of movement of the signal to be converted. Secondly, the associated counter disclosed is only a unidirectional binary scaler, capable of counting pulses of one type, corresponding, for example, to increases in current, but not decreases. To be of practical use, the converter would have to include another counter capable of detecting pulses of opposite polarity, for registering the decreasing steps of current. The values in the two counters would then need to be subtracted as each sample digital value was generated. Clearly, it would be desirable to avoid this additional complexity, and provide an A/D converter capable of producing a single digital output sample from a bidirectionally varying input signal. The present invention is directed to this end.